Conventionally, a positron emission computed tomography (PET) apparatus and an X-ray computed tomography (X-ray CT) apparatus have been known as a radiation diagnostic apparatus that use radiation. Such a radiation diagnostic apparatus presents images created on the basis of its own characteristics, and thereby realizes image diagnosis essential to today's medical practice.
A PET apparatus is one of nuclear medicine diagnostic devices, which offers detailed functional information on human body tissue in the form of images. More specifically, when a drug labeled with positron emitting radionuclides are introduced to a subject and positrons emitted from the introduced drug are bound to electrons and annihilated, the PET apparatus conducts coincidence counting on a pair of 511-keV gamma rays that are emitted substantially in opposite directions, by use of a detector having photon-counting detector modules arranged around the subject in the form of a ring. Then, the PET apparatus performs computations on the coincidence counting data of the gamma rays, and thereby reconstructs an image (PET image) that shows the distribution of the human body tissue into which the drug is introduced.
The coincidence counting process performed by the PET apparatus is now explained. First, in the PET apparatus, multiple Anger-type detector modules that each include scintillators in which NaI, BGO and the like are two-dimensionally aligned to convert the incident gamma rays to visible light and multiple photomultiplier tubes (PMTs) densely arranged by way of a light guide are arranged in the form of a ring (for example, see “Medical Image/Radiological Equipment Hand Book” edited by Japan Industries Association of Radiological Systems, published by Nago Bijutsu Insatsu Kabushiki Kaisha, 2001, pp. 190-191). The light guide is made of a light transmissive plastic or the like and used to transfer the visible light output by the scintillators to the PMTs. The PMTs multiply the visible light output by the scintillators and convert it to electric signals.
A coincidence circuit connected to the PMTs of each detector module generates coincidence counting information based on the results output by each detector module to determine the incident direction of a pair of gamma rays that are emitted from the positrons. More specifically, the coincidence circuit determines the incident position of the gamma rays in the detector module (i.e., the position of the scintillator) by calculating the position of the center of gravity from the positions of the PMTs that convert the visible light scattered from the scintillator to electric signals at the same timing and output the signals and the energy of the incident gamma rays corresponding to the intensity of the electric signals. In addition, the coincidence circuit integrates the intensity of the electric signal output by each PMT, and thereby calculates the energy value of the gamma ray that is incident on the detector module.
Then, the coincidence circuit performs a search (coincidence finding) for a combination of the results output by the detector modules, for example, in which the incident timing of the gamma ray falls within a specific time window width (e.g., 2 nanoseconds) and the energy values are within a specific energy window width (e.g., 350 keV to 550 keV). Then, the coincidence circuit generates coincidence counting information (coincidence list) as coincidentally counting information of two annihilation photons. Then, the PET apparatus uses the generated coincidence counting information as projection data (sinogram data), and reconstitutes a PET image by performing a back projection process on the projection data. In the coincidence circuit, random corrections can be made by use of a count ratio (count/sec) to eliminate the random coincidence that is included stochastically at a certain rate.
On the other hand, the X-ray CT apparatus is one of transmission CT apparatus, which offers detailed morphological information of human body tissue. More specifically, in the X-ray CT apparatus, the subject is irradiated with x rays from multiple directions by rotating the x-ray tube and the current-mode measuring detector in pair around the body axis of the subject, and the detector measures the intensity in different directions of the x rays that have been absorbed and attenuated when passing through the body. Then, by performing the back projection process on the projection data generated from the x-ray intensity distribution obtained by the detector, an X-ray CT image showing the morphological information of the human body tissue of the subject is reconstituted.
Moreover, recently, in the X-ray CT apparatus, a photon-counting CT that incorporates a photon-counting detector used in a PET apparatus or the like, in place of a conventional current-mode measuring detector, has been developed. In the photon-counting CT, each detection element of the photon-counting detector executes counting of the energy value of the X-rays that pass through the subject, and therefore a spectrum from which elements that constitute the body tissue of the X-rayed subject can be estimated can be prepared as projection data, and therefore an X-ray CT image describing differences in element level can be generated.
With a conventional PET apparatus, however, coincidence counting information generated only by a coincidence circuit that is a piece of hardware can be stored, which means that no coincidence counting information with a modified time window width or energy window width can be regenerated. In other words, with the conventional PET apparatus, the results output by detector modules are abandoned if they are determined as not coincident. For this reason, if a PET image needs to be corrected in response to a request from a reader of the PET image, for example, that the PET image should be reconstituted with a modified time window width or energy window width, a PET image has to be newly taken.
Furthermore, with the above photon-counting CT, only projection data is stored, but the counting result obtained by the photon-counting detector is not stored. Thus, image corrections such as scattered radiation corrections cannot be made by use of the counting results of the detector in response to a request from the reader of an x-ray CT image.
With the above conventional technologies, a medical image that is reconstituted by use of radiation cannot be quickly corrected in response to a request from a reader.